Re: [PATCH v1] mm,hwpoison: set PG_hwpoison for busy hugetlb pages

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Thu, May 12, 2022 at 10:54:05AM +0800, Miaohe Lin wrote:
> On 2022/5/12 2:35, Mike Kravetz wrote:
> > On 5/11/22 08:19, Naoya Horiguchi wrote:
> >> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> >>
> >> If memory_failure() fails to grab page refcount on a hugetlb page
> >> because it's busy, it returns without setting PG_hwpoison on it.
> >> This not only loses a chance of error containment, but breaks the rule
> >> that action_result() should be called only when memory_failure() do
> >> any of handling work (even if that's just setting PG_hwpoison).
> >> This inconsistency could harm code maintainability.
> >>
> >> So set PG_hwpoison and call hugetlb_set_page_hwpoison() for such a case.
> 
> I'm sorry but where is hugetlb_set_page_hwpoison() defined and used ? I can't find it.

Sorry, this depends on the unmerged patch
https://lore.kernel.org/linux-mm/20220427042841.678351-2-naoya.horiguchi@linux.dev/
, so should come after that.  I'll do both into a single patchset next.

...
> >> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> >> index 6a28d020a4da..e3269b991016 100644
> >> --- a/mm/memory-failure.c
> >> +++ b/mm/memory-failure.c
> >> @@ -1526,7 +1526,8 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
> >>  			count_increased = true;
> >>  	} else {
> >>  		ret = -EBUSY;
> >> -		goto out;
> >> +		if (!(flags & MF_NO_RETRY))
> >> +			goto out;
> >>  	}
> > 
> > Hi Naoya,
> > 
> > We are in the else block because !HPageFreed() and !HPageMigratable().
> > IIUC, this likely means the page is isolated.  One common reason for isolation
> > is migration.  So, the page could be isolated and on a list for migration.
> > 
> > I took a quick look at the hugetlb migration code and did not see any checks
> > for PageHWPoison after a hugetlb page is isolated.  I could have missed
> > something?  If there are no checks, we will read the PageHWPoison page
> > in kernel mode while copying to the migration target.
> > 
> > Is this an issue?  Is is something we need to be concerned with?  Memory
> > errors can happen at any time, and gracefully handling them is best effort.
> 
> It seems HWPoison hugetlb page will still be accessed before this patch. Can we do a
> get_page_unless_zero first here to ensure that hugetlb page migration should fail due
> to this extra page reference and thus not access the page content? If hugetlb page is
> already freezed, corrupted memory will still be consumed though. :(

Right, I have no idea about this ...

Thanks,
Naoya Horiguchi